Among various Silicon on Insulation (SOI) approaches SIMOX (separation by oxygen implantation) seems to be the most promising as a replacement for use of silicon on sapphire (SOS) in CMOS processing. During the SIMOX process oxygen at the dose of about 1.0-2.0.times.10.sup.18 oxygen atoms cm.sup.-2 is implanted into a silicon wafer with energy of about 150-200 Kev, within the ideal temperature range of from about 450.degree. C. to about 650.degree. C. To remove the implantation damage and to form a buried oxide, the SIMOX wafers are subsequently annealed at a temperature in the range from 1150.degree. C. to 1400.degree. C. A typical SIMOX structure is shown in FIG. 1 wherein a wafer 10 includes an oxygen implanted layer 12 and a regrown SIMOX film 14. A thin surface layer (.congruent.10-20 nm) of the SIMOX wafers serves as a seed during the solid state regrowth process. The degree of crystalline perfection of this layer will influence the crystallographic perfection of the regrown SIMOX film 14.
While the implantation dose and voltage, which determine the thickness of both the buried oxide layer and the top silicon layer are important, they are somewhat easily monitored and controlled. The implantation temperature, on the other hand, is of crucial importance and is difficult to monitor and, therefore, difficult to control. It is well known that the crystallographic structure of the SIMOX films is determined by the implantation temperature, especially in the formation of a polysilicon electric field shielding layer, after post-implantation annealing. Also, the formation of thermal donors is related to this temperature. Adverse effects which occur when the implantation temperature substantially deviates from the ideal range of temperatures will remain in the film even after subsequent processing. For example, where the substrate temperature falls below about 440.degree. C., amorphism will occur in SIMOX film. On the other hand, when the temperature exceeds about 670.degree. C., the SIMOX film will become polycrystalline.
The determination of the substrate temperature used during implantation, at a time subsequent to actual implantation is difficult. However, there is a need for such a determination which will enable the selection of only high quality SIMOX wafers for further processing. This will greatly increase yield and reliability of completed devices and substantially reduce costs.
SUMMARY OF THE INVENTION
The present invention includes a plurality of graphs and a method for utilizing the graphs for examining a SIMOX wafer to determine the substrate temperature used during oxygen implantation. Each of the graphs represent a relationship between a wavelength of minimum transmittance of infrared radiation through a SIMOX wafer and the substrate temperature used during oxygen implantation of the wafer. The graphs may take the form of plots on a chart or a representation of the information in the graph retrievably stored in a computer accessible memory. Further, each graph is related to a particular dose of oxygen and energy level. Infrared energy having a spectrum of wavelengths spanning the range of from about 1000 cm.sup.-1 to about 1100 cm.sup.-1 is transmitted through a portion of the SIMOX wafer and the wavelength of minimum transmittance is determined. The dose of oxygen and energy used during oxygen implantation of the SIMOX wafer is determined and a graph corresponding to these two parameters is selected. The substrate temperature corresponding to the wavelength of minimum transmittance is determined as indicated by the graph.